In the manufacture of integrated circuits, the technique of photolithography is frequently used. In the practice of this technique, a semiconductor wafer is coated with a photoresist, which is then exposed with ultraviolet radiation which is passed through a mask so that a desired pattern is imaged on the photoresist. This causes changes in the solubility of the exposed areas of the photoresist such that after development in a suitable solvent the desired pattern is fixed on the wafer, whereupon the photoresist may be hardbaked to enable it to withstand subsequent processing.
In such subsequent processing, integrated circuit components which correspond to the desired pattern are formed by processing including plasma etching or ion implantation.
After the integrated circuit components are formed, it is desired to strip the photoresist from the wafer, which at this point has already served its useful purpose. The relative ease or difficulty with which the photoresist may be stripped depends on the degree to which physical and chemical changes have been induced in the resist during the specific plasma etching or ion implantation processes and on the degree to which the resist has been cross-linked. Thus, it is generally known that a significant degree of hard baking and to an even greater extent, the processes of plasma etching and ion implantation induce physical and chemical changes in the photoresist, so that stripping is particularly difficult.
U.S. Pat. No. 4,885,047 to Ury et al which is assigned in common with the instant invention, teaches an apparatus for stripping resist from a semiconductor wafer wherein a planar member is established relative to a wafer to form a small gap therebetween, an oxidant such as ozone is passed at high speed along the wafer, and then the oxidant gas stream including oxidized resist is exhausted from the chamber. Such an apparatus is sometimes referred to as an asher.
U.S. Pat. No. 5,071,485 to Matthews et al which is also assigned in common with the instant invention teaches an improvement of the asher apparatus taught by Ury et al wherein the oxidant gas is supplied at the periphery of the wafer, flows radially inward with respect to the wafer, and is exhausted from above the center of the wafer. U.S. Pat. No. 5,071,485 to Matthews et al is incorporated herein by reference.
FIG. 1 is a schematic representation of the exhaust system of an actual asher of the type described in the above-identified patents. The ashing process is conducted in the processing chamber 1, wherein ozone is passed through a narrow gap as shown in the figure which overlies the resist. The ozone which oxidizes the resist in the ashing process is exhausted through a central orifice 2. The central orifice 2 is connected via a fitting 3 to a teflon exhaust conduit 4. The exhaust conduit 4 is connected via a second fitting 4a to an ozone destroyer 5. The ozone destroyer, 5 is necessary because all of the ozone is not used up in the processing chamber 1 and it is unsafe to release ozone. The ozone destroyer 5 comprises a vessel with an inlet and outlet which is filled with aluminum pellets so as to provide a large surface area of aluminum which is effective in reacting ozone into oxygen. The outlet 6 of the ozone destroyer 5 is connected to a second conduit. Most of the exhaust gas flows along the second conduit 7 to an ejector 8 and in turn to the plant facility exhaust (not shown) through final conduit 9. A small portion of the exhaust gas exiting from the ozone destroyer 5 through exit 6 is diverted to a branch conduit 7A and flows to a cooling heat exchanger 10 and then through a carbon dioxide content monitor Il and finally to a second ejector 12. The carbon dioxide level in the exhaust indicates the resist oxidation rate and is used to monitor the ashing process.
This type of asher suffers from the problem that the resist, although oxidized to a sufficient degree to be broken free of the wafer, is not completely oxidized. Molecular and larger resist fragments are carried into the exhaust stream. These fragments tend to deposit and clog the system. In the particular system described above the fragments tend to deposit primarily at the fitting 4a leading into the ozone destroyer 5. Fouling also occurs at the exit port 6 of the ozone destroyer and at the heat exchanger 10.
The type of resist removal machine described above uses an oxidant gas at about atmospheric pressure. Other types of resist removal machines use low pressure gas and do not suffer so severely the problem of resist fouling in the exhaust tract. Of course the low pressure ashers have their own set of problems, an important one of which is that they can cause damage by bombarding the wafer with energetic species.